Accelerator target positioner



July 2, 1957 H. G. HEARD ETAL 2,798,178

ACCELERATOR TARGET POSITIONER Filed May 17, 1956 3 Sheets-Sheet l RD N s Z I 4 j my /fi N I Q 2 m 4 i, 3 I III h INVENTORS. HARRY 'G. HEARD Y WILL/'AM W. SALS/G, JR.

ATTORNEY.

July 2, 1957 H. G. HEARD ETAL 2,798,178

ACCELERATOR TARGET POSITIONER w T m N) r\ w V, a LEM N) vv E SI "L. 5g 5.9 INVENTORS.

'5 5g HARRY G. HEARD O 0 BY WILL/AM W. SALSIQJR A TTORNEX July 2, 1957 Filed May 1'7, 1956 H. G. HEARD Fl AL ACCELERATOR TARGET POSITIONER 3 Sheets-Sheet 3 3i U} .J'

J 0 2 W 1 .M- 5 /8J 24I I 25) INVENTORS.'

HARRY G. HEARD BY WILL/AM W SALs/G, JR. %Ma 4% ATTORNEY I Unite f ttes Patent AQCELERATOR TARGET PQSETIQNER Harry G. Heard and Wiliiatn W. Salsig, In, Berkeley, Calif., assignors to the United States of America as represented by the United States Atomic Energy Commission Appiication May 1'7, 1956, Serial No. 535,584

18 Claims. (Cl. 313-62) This invention relates to charged particle accelerators and, more particularly, to apparatus employed therewith for the timed insertion of target substances into the path of the particle beam.

The extraction of high energy beams from certain forms of charged particle accelerators, a proton synchrotron for example, is a complex procedure requiring bulky and costly equipment. Where such extraction of the beam is carried out, it is usually possible to extract only a small proportion of the beam which circulates within the accelerator, thus reducing the available beam intensity by a very large factor. Accordingly, it is desirable, where conditions permit, to mount the substance to be irradiated within the accelerator itself.

In the case of an accelerator having a beam orbit which is substantially constant in radius throughout the accelerating cycle, as in a synchrotron or proton synchrotron, a target obviously cannot be positioned in the beam trajectory until the acceleration of a pulse of particles is completed, at which time it must be rapidly inserted to obtain maximum irradiation. As the life of an individual beam pulse is short, about one and three quarter seconds in the University of California bevatron, the target in sertion apparatus must operate quickly and precisely. Since the beam is of comparatively great cross-sectional diameter during the initial phase of acceleration, the target must have a retracted position at a considerable distance from the median beam orbit. It is apparent that the mechanism should be adapted to insert target substances of widely varying mass and should come to rest with a minimum of whiplash.

It is accordingly a principal object of this invention to provide means facilitating the irradiation of substances within the beam accelerating region of a particle accelerator. 7

It is another object of this invention to provide a target mounting for the rapid positioning of a target substance in the beam of a particle accelerator.

It is an important object of this invention to provide means for rapidly inserting a target substance into the beam orbit of a particle accelerator, which insertion is characterized by the absence of whiplash.

Still another object of this invention is to provide an accelerator target mounting which interposes a target substance within the beam orbit of the accelerator at a precise and fixed position and at a desired instant in the accelerating cycle.

A further object of the present invention is to provide a target positioner for a charged particle accelerator, which target positioner minimizes disturbance of the magnetic field of said accelerator.

The invention, both as to its organization and method of operation together with further objects and advan 2,798,178 Patented July 2, 1957 tages thereof, will best be understood by reference to the following specification taken in conjunction with the accompanying drawing, of which:

Figure l is a plan view of one embodiment of the invention shown mounted Within the vacuum envelope of' a proton synchrotron and with the target in retracted position;

Figure 2 is an elevation view of the apparatus of Fig. 1;:

Figure 3 is a sectional elevation view taken along line- 3-3 of Fig. 1;

Figure 4 is an elevation view of the target mounting arm of Fig. 1; and

Figure 5 is a view of a portion of the apparatus modified to position an elongated target substance.

Referring now to the drawing and more particularly to Figs. 1 and 2 thereof, the invention is shown mounted Within the vacuum envelope 11 of a proton synchrotron. As is well known within the art, the vacuum envelope of this class of particle accelerator has a generally toroidal configuration conforming to the annular magnet employed. In some instances the annular magnet structure may be broken into quadrants which are physically spaced apart and joined by straight sections of vacuum envelope to form a continuous beam accelerating chamber. In these latter cases, the present invention must be mounted within or adjacent one of the quadrants inasmuch as the invention makes use of the magnetic field of the accelerator.

The embodiment of the invention to be herein described was designed for use in the University of California bevatron in which the vacuum envelope 11 has a rectangular cross-section, the apparatus being disposed on the floor of the envelope radially inward from the region occupied by the circulating particle beam 12. It will be apparent however that other positions are possible, particularly in the base plate, coaxial with the fasteners, in order to pro-- vide a gap beneath the plate for the passage of such cooling coils, magnet windings or other facilities as may be;

required. It will be appreciated that the particle beam varies in cross-sectional area, being relatively large dur-- ing the initial portion of the acceleration cycle as indicated by the dashed beam profile 17, and narrowing to the much smaller cross-section 12 during the terminal period of the cycle. Accordingly, in fixing the position of the base plate 13, as well as other components of the invention, ample clearance from the maximum beam cross section 17 must be provided.

A second smaller base plate 18 is similarly disposed on the vacuum tank fioor by means of fasteners 1% and spacers 21 and is positioned radially outward from the first base plate 13 and a slight distance radially inward from the edge of the minimum beam orbit 12. An elongated rotatable shaft 22 is mounted over the two base plates 13 and 18, transverse to the beam orbit 12, by bearings 23 one secured to each of the base plates. The bearings 23 are spaced apart a distance less than the total length of the shaft 22 so that the extremities of the shaft project through the bearings.

An elongaed target mounting arm 24 projects from the shaft 22., at right angles thereto, at a position immediately beneath the inside edge of the final particle orbit 12, the arm thus being adapted to swing a target from a position adjacent the floor of the vacuum tank to a position where;

- w rm the circulating high energy particle beam is intercepted.

Transversely "secured to"tlie"fr'e end" of i the target arm 24 is a platform 25 upon which a substance 26 to be irradiated may be placed and secured by suitable means as by an adhesive layer 27, for example.

Means controlling rotation of the shaft 22, and thus controlling motion of the target arm 24, comprises an electrical rotor assembly 28 mounting a flat multi-turn electrical coil or winding 29. The coil 29 is, in this embodiment, wound in'a generally rectangular eenfi ura-tien and-afiixed to a rectangulardielectricstiffener frame 31 by a wrapping of insulating tape 32; An axle 33 bisects the coil assembly'28, passing through diametrically opposed apertures inthe stifiener'" frame- 3'1, the ends of the vertical, "as indicated by field lines 35 of Fig. 2, and the axle 33 is horizontal and positioned above, olfset from, and parallel to the shaft 22. i i

T heend'of the axle 33 most adjacent the beam orbit 12 is projected a short distance through the bearing aperture 37 of the post 36 and a crank arm 38 is radially secured thereto. A second shorter crank arm 39 is radially secured to the shaft '22 at a longitudinal point thereon corresponding toth'e plane of rotation of the first crank arm 38.' To transfer rotation ofthe coil assembly 28 to the target arm 24, the two crank arms 38 and 39 are connected by a pair of parallel rigid links 41 and 41" which are secured'to the extremities of the two crank arms by transverse pivot pins 42 and 42'. I

To' damp any tendency towards vibration or whiplash in the moving elements described above, a damping vane '43, interacting with the magnetic field of the acceleratorj is projected radially from the shaft 22 at the extremity thereof furtherest from the beam orbit 12. The

vane 43, formed of electrically conducting material, has

, a generally triangular shape with the apex closest to the shaft 22, and projects from the shaft in the plane defined by the shaft and the targetar'm 24 in'order that maximum coupling with the magnetic field will occur as the .target arm swings to a position nearest the beam orbit.

To limit motion of the target arm 24 and damping vane 43 to a ninety degree arc, a resilient stop 44 backed by an angled-bracket 46 projects upward from the first base plate 13, the bracket and attached stop being posii tioned to be struck by the vane when the latter reaches a vertical position. Motion of the' target arm 24and vane 43 below a horizontalposition is prevented'by a second resilient stop 47 secured between the two upright posts 60 34 and 36 by means of a crosspiece 48, the'stop being positioned to prevent the coil stiffener 31 from rotating past a vertical position. I

To provide a remote indication, external to the accelerator chamber, of the position of the target arm 24, a trip rod 49 is secured to the first crank arm 38 in a downwardly projecting relationship. -A pair of spaced micro switches 51 and 51' are mounted on the first base plate 13 at positions corresponding tothe limits'of travel of the lower end of'the'tr'ip arm'where the actuatorbuttons" 52 and 52 of the microswitches 'arh contacted by the trip rod when it is at either of its extreme positi apparent thef r'r iicroswitches 51 and "51 rnay'be connected I to operate indicator "lamps on a remotely situated fc'bnifglj i a, i f ti i rhft n thepanel, th

4 target substance 26 is retracted or in a beam intercepting I posifidfl'.

In operation, the apparatus being positioned within the particle accelerator as previously described, movement of the target substance 26 into and away from the particle beam 12 is efiected by controlling the energization supplied to the coil 29. Such energization is provided through leads 53 which are wrapped around the pivot axle 33 to avoid entangling with the mechanism and which are passed to a source of direct current 54 through a control switch 56. It is apparent that the latter elements, current supply 54 and control switch 56, may take a wide variety of forms according to the structure of the associated accelerator and are therefore here shown schematically. It will generally be'found most satisfactory to key the switch 56 to some portion of the accelerator control system in order that thet'arg'e't'26 will be inserted and're -T tracted from the beam at the optimum moments. The action of the switch 56 is such that at the desired portion of the beam accelerating cycle, generally when the beam has reached maximum energy, a current is supplied to the coil 29, the current being maintained for a desired interval andthen reversed to retract the target.

The jniagnetidfield'of' the energized coil 29 interacts with the magneticfield 35'of the accelerator to rotate the a coil assemblythrough a' ninety degree arc, furthenrota tion of the assembly being limited by the vane abutting against thest'op' 441 IRotationbf the coil assembly 28 is transferred to the target and 24 th rough the pivot axle 33,

shafti22.,i'iReversalf of thecoil current by the control switch counter rotates the coil assembly 28 to the I original position and th ql r h the de scribed linkage retracts the target 24 Considering now the acti on ofthe damping vane 43 in smoothing the motion of the assembly and in minimizing vibration and whiplash it maybe seen that the vanejr0 tates ini a reverse manner from the rotation of the coil assent b11 28, fthatjis thevane moves from a horizontal position to a vertical oneas the coil moves from vertical to horizontal, ;As the coil d'escribes the latter motion, the torqiielfopce producing such movement, which torque is initially strong, dec rea s es as a cosine function of the angle traversed, Concurrent with such action, the vane 43 icutsjthro ugh the magnetic field of the accelerator at a rate increases as a sine function of the angle traversed 'As the vane 43 rotates, eddy currents are created which through their associated magnetic fields, increasingly oppose further motion of the apparatus.

Thus the vane 43, opposing the decreasing rotational forces actingon the coil assembly 28 provides an increasing damping influence which has a maximum effectv in the last stage of the movement therefor tending to bring the apparatus gently'to rest in' the target up position. In practice, the;action is prematurely stopped by the vane. 43 coming into abutmentagainst the stop block 44, where-' upon the eddy currents in the vane are rapidly dissipated and the assemblage is heldin position by the remaining rotational tendency of the coil assembly 28 which persists until current to the [coil is stopped or reversed.

It will be apparent thatinterchangeable damping vanes 43 of varying sizes jmight be provided, and that the current strengthsupplied the coil ,29. may be varied, to adjust for differences in the mass of various targets 26. 1v Such adjustmentmay be particularly desirable when e'm-' ploying elect1fically conductive target substances as the target will then exhibit a damping influence analogous to that of the vane '43. To prevent further unwanted damping effects, as well as to minimize disturbance of the accelerator magnetic field, it willbe found preferablev to formall elements ofthe invention, with the exception of the, damping vane ,43and the electrical components," of dielectric material. l 'In some instancesit may be desirable to irradiate a target whichis elongated in the direction of the beam and must therefore remain horizontal while being raised and lowered. Referring now to Fig. there is shown a modification of the described embodiment by which this may be accomplished. The elements of this apparatus are similar to those of the previous embodiment except as willnow be described.

The second baseplate 18 must in this instance be widened so that the bearing block 23 and the shaft 22 are located centrally upon the plate. A second shaft 22', mounted by a second bearing block 23', is disposed over the baseplate 18 parallel with the first shaft 22. The second shaft 22' is provided with a radial crank arm 39', similar to the crank arm 39 on the first shaft. A link 58 pivotally connects the two crank arms 39 and 39' in order that the two shafts 22 and 22' will rotate in unison. The target arm 24 differs from that of the previous em bodiment by having a right angled lower extremity 59, the point of connection with shaft 22 thus being offset from the axis of the arm for a purpose which will hereinafter be made clear. The target retaining platform 25' is elongated in the direction of the ion beam to support the long target 26' and is pivotally fastened at the inside edge to the upper extremity of the target arm 24' by a transverse pin 61. v

To stabilize the target and hold it in a horizontal position at all times, a second target arm 62 projects radially from the second shaft 22, the second arm being parallel to the first arm 24' and having a right angled upper extremity 63 projecting in the direction of the first arm. A second pivot pin 61' attaches the angled extremity 63 of the second target arm to the target retain ing platform 25.

.For the most satisfactory operation, the pivoted juncture 61 of the first target arm 24' with the target retaining platform 25 should be beneath the approximate center of gravity of the target 26, the second or stabilizing target arm 62 being more proximal to an extremity of the target. As may be seen, rotation of the crank arm 38, through the action of the coil assembly as previously described, will rotate the two target arms 24'. and 62 lifting the target from a retracted position, as shown in phantom in Pig. 5, to the beam intercepting position. As may be observed, the efiect of the angled extremities 59 and 63 of the target arms 24 and 62, respectively, is to allow the arms to nest together in the retracted position so that maximum lowering of the target may be achieved.

It will be apparent to those skilled in the art that numerous modifications, other than those herein disclosed, are possible within the spirit and scope of the invention and thus it is not intended to limit the invention except as defined in the following claims.

What is claimed is:

1. In a target mounting for a fixed beam orbit charged particle accelerator, the combination comprising a base element secured within the vacuum chamber of said accelerator, an elongated target arm having a first extremity pivotably fastened to said base element, the second extremitiy of said arm being swingable in the direction of said beam orbit, said arm having provision for retaining a target substance on said second extremity thereof, an electrical winding movably mounted within the magnetic field of said accelerator, a controlled direct current source supplying energization to said winding, and means coupling motion of said winding to said arm whereby said target substance is swung in the direction of said beam orbit.

2. A target mounting as described in claim 1 and having a motion damping means comprising an electrically conducting member of substantial area pivotably mounted within said magnetic field and constrained to pivot across said field concurrent with motion of said arm.

'3. A target mounting as described in claim 1 wherein said electrical winding comprises a closed loop mounted to rotate about a diameter thereof, and said motion couand linked to said target arm.

4. 'In a target mounting as described in claim 2 wherein said provision for retaining a target substance to said arm comprises a flat platform pivotally mounted on the extremity of said arm, the further combination of a stabilizing arm having a first extremity pivotally fastened to said base element and a second opposite extremity pivotally fastened to said platform, said stabilizing arm being mounted parallel with said target arm.

5. In a target positioner for a fixed beam orbit electromagnetic particle accelerator, the combination comprising a base element mounted within the vacuum tank of said accelerator, a target arm having a first extremity pivotably afiixed to said base element, the second extremity of said arm being swingable in the direction of said beam orbit, target securing means disposed on said second extremity of said arm, a movable electrical coil mounted within the magnetic field of said accelerator, a source of reversible direct current controllably supplying energization to said coil, means transferring motion of said coil to said target arm, an electrically conducting damping element of substantial area movably disposed within said magnetic field, and means linking motion'of said damping element and said target arm obtaining simultaneous motion of the :two elements.

6. An accelerator target positioner as described in claim 5 wherein said damping element is disposed within the plane defined by said target arm and the pivot axis thereof.

7. A target mounting for use within the vacuum vessel of a fixed beam orbit charged particle accelerator, comprising a base member secured within said vacuum vessel, an elongated target arm having one extremity pivotably fastened to said base member, the opposite extremity of said arm being positioned to pivot in the direction of said beam orbit and being provided with means for retaining a substance which is to be irradiated, an electrical winding positioned within the magnetic field of said accelerator, controlled current supply means exciting said winding, a conducting damping vane pivoted at one extremity, and linkage means simultaneously pivoting said target arm and I said damping vane upon movement of said winding. r

8. A target mounting substantially as described in claim 7 whereinsaid damping .vane pivots through substantially a ninety-degree arc, said damping vane being positioned to be directed along said magnetic field when said target arm is closest to said beam orbit, and transverse to said magnetic field when said target arm is most removed from said beam orbit.

9. A target mounting for a charged particle accelerator comprising an elongated target arm disposed within the vacuum vessel of said accelerator and fastened to pivot about one extremity into and away from the beam orbit of said accelerator, an electrical winding in the form of a closed loop, at least one bearing element supporting said loop for rotation about a diameter thereof, a controllable current source supplying excitation to said winding, and linkage transferring rotation of said winding to said target arm.

10. A target mounting as described in claim 9 and further characterized by an electrically conducting damping vane pivotally mounted at one extremity within said magnetic field, said vane being constrained to rotate with said winding and being oriented substantially normal to the plane of said winding.

11. A particle accelerator internal target positioner comprising a rotatable shaft disposed within the beam chamber of said accelerator, an elongated target arm projecting radially from said shaft, the extremity of said target arm having provision for the attachment of a target substance, a conducting motion damping vane projecting radially from said shaft and having a substantial area Within the plane defined by said target arm and said shaft, an electrical coil mounted to rotate in the magnetic field of said accelerator, linkage transferring rotation of said coil to said shaft and a controlled direct current supply energizing said coil.

"122 1 A target pnsi'ti'oner asdescribed in claim} 11 wherein 'saidrprovision fcir the attachment of -a' target substance to'asaid target' armcomprises a platform pivotall'y secured to the extremity ofsaida'rm, and comprising the further combination of *a'sejcond 'rotatableshaft disposedparallel to said first shaft 'and' linked to said first'shaf'tto rotate concurrently:therewith, and a second"elongate'd arm-projecting radially from said second shaft and being pivotally fastened to 'saidplatform, said second arm-being parallel tosaid'first arm.

l3.' A target positioner for a charged particle accelerator'comprising arotatable shaft' 'fixed within'the vacuum chambenof said accelerator transverse to the beam orbit-thereof and offset from saidbeam-orbit, an elongated target armprojecting-radially from saidshaft at the extrmitydh'ereoflmost:proximal to said'beam orbit, said arm having-provision for retaining a-substance-to be irradiated,.an electrical Winding, at leas't one bearing element mountingi'said winding for rotatior r abouta diameter thereof; means linking rotation of'said'winding to said shaft a current supply energizinglsaid"winding, and an electrically conducting damping'vane projecting radially from'sa'idshaft, said vane lying in the plane defined by said shaft and said arm.

14.:A target mountingfor a charged-particle accelerator comprising a rotatable'shaft mountedw'ithin'the beam chamber of said acceleratortransverse to the beam orbit thereinand offset fromsaid beam orbit, an elongated target varm projectingradially from said-shaft, the free extremity of-said arm having 'provision for-the'attachment of a' substance to be irradiated, a fiat electrical winding having two journals affixed at diametrically'opposed points thereon, abearing mounting engaging'saidjournals to support 's'aid winding for rotation in the magnetic field-of said accelerator, linkage transferringrotation 'of 'said' 'journals to. said sh'aft, a motion dampingvane projecting radially from said'shaft at an azimuth similar to that of said arm, and a source of reversible direct cu rrentconnected to said coil.

15. A target mounting for acharged particle accelerator as described in claim 14 wherein said linkage comprises a first crank'arm projecting'radially from one of said journals, a second crank arm-projecting radially from said shaft and at' least one rigid link having a first extremity pivotably secured to the free end of said first crank arm said second crank arm.

8 16. A- target mounting -as described-'=inclaim-14"vvl'1erein said provision for= the attachment of 'a -sub'stancetybe irradiatedto-said arm-comprises aplttforni'the central portion thereof'being'pivotally fastened to the-"extremity of: said arm; and-comprising the further combination of a second rotatable shaft disposed parallel tosaicl firs't shaft and linked'to said first shaft for 'rotation inunis'on therewith, and a stabilizing arm projecting radially 'from" said second'sh'aft in parallel re'lationship with s'aid target arm, said "stabilizing arm being pivot ally'-'co'nnected Witl-l said platform.

17. Apparatus for the'time-insertionof a target substance intothe circulating beam of a'chargedparticleaccelerator comprising at leastone-flatbase-plate-disposed on the floor of the beam" accelerating envelope'o'f' said particle accelerator, an elongated'li0rizohtal shaft'disposed immediately above said base I member -andaligned Lalng a radius of said accelerator, apI'urality-Of bearings spaced along said shaftsecuring said shafttosaid base 'pl'atqa target monnting-a'rmprojecting radially from-said' s'h aft at a point thereon substantially beneath said circulating particle beam, a pair otiupr'ight posts secured to'said-base plate and spaced apart along a'line paralleling-saidshaft, each said po'st having an aperture in the'u'pper extremity thereof, a multi-turn electrical 'coil' mountedl betweensaid upright posts by diametrically opposed jQurnaIsseatedin saidapertures in saidposts, -a first crankarm radiallyprojected from one of'said journals, a second"crank' arm radially projected from said shaft; rigid link mans pivotally connected 'at'one' extremitydo:sai'd first' crank arm and pivotallyconnected at the-opposite extremity tosaid second crank arm, means supplying timed reversibledirect current excitation to said-coil, and a'conducting- 'damping vane projected radially from said'shaft' in the plane definedby said-shaft "and said arm.

'18. Apparatus as described in claim l 5 'an'd comprising Livingood et al -Nov.- 21, 1950 Livingston e Jan.'3,-1952 Kerst= Nov. 23,- 1943 

